Something changed while I was building a worksheet. I started out able to use center across selection and the cell content would spread over the columns included in the selection. Now only the amount that will fit in the first cell shows and the remainder is hidden. Merge & center accomplishes what I want, but comes with restrictions on what happens to other cells in the same columns. What did I do to change how center across selection worked and how may I undo it?
It's hard (I'm tempted to say "impossible") to determine what you did that is causing the symptoms you describe.
In general, however, the advice in all the books and sources of expert advice is to stay away from merging and centering until you've got the final functionality of the workbook firmly established. Said another way, make sure it works as it's supposed to work before you make it look like you want it to look.
Early attention paid to the aesthetics, to color and headings and all, often can interfere with functionality.
That's not the answer you are wanting to hear, I'm quite sure, but I thought you at least deserved some kind of response after more than 50 views. Feel free to ignore it and wait for other thoughts.